A downhole tool comprises a housing defining a main bore and an actuatable member moveable from a first configuration to a second configuration to permit actuation of the tool. The tool includes an indexer mounted within the housing and arranged to progress linearly in a first direction along the main bore of the housing in a predetermined number of discrete sequential and progressive steps of linear movement in said first direction by passage of a corresponding number of objects through the indexer to cause said actuatable member to move towards its second configuration.